review relational query optimization

Relational Query Optimization

CS 186, Fall 2002, Lecture 20 R & G Chapter 13

It is safer to accept any chancethat offers itself, and extemporize a procedure to fit it, than to get a good plan matured, and waitfor a chance of using it.

Thomas Hardy (1874)in Far from the Madding Crowd

Review

• Implementation of single Relational Operations

• Choices depend on indexes, memory, stats,…• Joins

– Blocked nested loops:• simple, exploits extra memory

– Indexed nested loops:• best if 1 rel small and one indexed

– Sort/Merge Join• good with small amount of memory, bad with duplicates

– Hash Join• fast (enough memory), bad with skewed data

Query Optimization Overview

SELECT S.snameFROM Reserves R, Sailors SWHERE R.sid=S.sid AND

R.bid=100 AND S.rating>5

Reserves Sailors

sid=sid

bid=100 rating > 5

sname

• Query can be converted to relational algebra• Rel. Algebra converted to tree, joins as branches• Each operator has implementation choices• Operators can also be applied in different order!

π(sname)σ(bid=100 ∧ rating > 5) (Reserves a` Sailors)

Iterator Interface• A note on implementation:

•Relational operators at nodes support uniform iterator interface:

Open( ), get_next( ), close( )•Unary Ops – On Open() call Open() on child.

•Binary Ops – call Open() on left child then on right.

•By convention, outer is on left.Reserves Sailors

sid=sid

bid=100 rating > 5

sname

Alternative is pipelining (i.e. a “push”-based approach).

Can combine push & pull using special operators.

Query Optimization Overview (cont)

• Plan: Tree of R.A. ops, with choice of algorithm for each op.– Each operator typically implemented using a `pull’

interface: when an operator is `pulled’ for the next output tuples, it `pulls’ on its inputs and computes them.

• Two main issues:– For a given query, what plans are considered?

• Algorithm to search plan space for cheapest (estimated) plan.

– How is the cost of a plan estimated?• Ideally: Want to find best plan. • Reality: Avoid worst plans!

Cost-based Query Sub-System

Query Parser

Query Optimizer

Plan Generator

Plan Cost Estimator

Query Plan Evaluator

Catalog Manager

Usually there is aheuristics-basedrewriting step beforethe cost-based steps.

Schema Statistics

Select *From Blah BWhere B.blah = blah

Queries

Schema for Examples

• As seen in previous two lectures…• Reserves:

– Each tuple is 40 bytes long, 100 tuples per page, 1000 pages.

– Let’s say there are 100 boats.• Sailors:

– Each tuple is 50 bytes long, 80 tuples per page, 500 pages.– Let’s say there are 10 different ratings.

Sailors (sid: integer, sname: string, rating: integer, age: real)Reserves (sid: integer, bid: integer, day: dates, rname: string)

Motivating Example

• Cost: 500+500*1000 I/Os• By no means the worst plan! • Misses several opportunities:

selections could have been `pushed’ earlier, no use is made of any available indexes, etc.

• Goal of optimization: To find more efficient plans that compute the same answer.

SELECT S.snameFROM Reserves R, Sailors SWHERE R.sid=S.sid AND

R.bid=100 AND S.rating>5

Sailors Reserves

sid=sid

bid=100 rating > 5

sname

(Page-OrientedNested loops)

(On-the-fly)

(On-the-fly)Plan:

500,500 IOs

Alternative Plans – Push Selects (No Indexes)

Sailors Reserves

sid=sid

bid=100 rating > 5

sname


(On-the-fly)

(On-the-fly)

Sailors

Reserves

sid=sid

rating > 5

sname


(On-the-fly)

(On-the-fly)

bid=100 (On-the-fly)

250,500 IOs


Sailors

Reserves

sid=sid

rating > 5

sname


(On-the-fly)

(On-the-fly)


Sailors Reserves

sid=sid

bid = 100

sname


(On-the-fly)

rating > 5

(On-the-fly)(On-the-fly)

250,500 IOs250,500 IOs

Sailors

Reserves

sid=sid

rating > 5

sname


(On-the-fly)

(On-the-fly)


6000 IOs

Sailors

Reserves

sid=sid

rating > 5

sname


(On-the-fly)

(On-the-fly)

bid=100

(On-the-fly)

250,500 IOs


SailorsReserves

sid=sid

rating > 5

sname


(On-the-fly)

bid=100 (Scan &Write totemp T2)(On-the-fly)

6000 IOs

Sailors

Reserves

sid=sid

rating > 5

sname


(On-the-fly)

(On-the-fly)

bid=100

(On-the-fly)


4250 IOs1000 + 500+ 250 + (10 * 250)

ReservesSailors

sid=sid

bid=100

sname


(On-the-fly)

rating>5 (Scan &Write totemp T2)(On-the-fly)


4010 IOs500 + 1000 +10 +(250 *10)

SailorsReserves

sid=sid

rating > 5

sname


(On-the-fly)

bid=100 (Scan &Write totemp T2)(On-the-fly)

4250 IOs

Alternative Plans 1 (No Indexes)

• Main difference: Sort Merge Join

• With 5 buffers, cost of plan:– Scan Reserves (1000) + write temp T1 (10 pages, if we

have 100 boats, uniform distribution).– Scan Sailors (500) + write temp T2 (250 pages, if have 10 ratings).– Sort T1 (2*2*10), sort T2 (2*3*250), merge (10+250)

– Total: 3560 page I/Os.• If use BNL join, join = 10+4*250, total cost = 2770.• Can also `push’ projections, but must be careful!

– T1 has only sid, T2 only sid, sname:– T1 fits in 3 pgs, cost of BNL under 250 pgs, total < 2000.

Reserves Sailors

sid=sid

bid=100

sname(On-the-fly)

rating > 5(Scan;write to temp T1)

(Scan;write totemp T2)

(Sort-Merge Join)

Alt Plan 2: Indexes• With clustered index on bid

of Reserves, we get 100,000/100 = 1000 tuples on 1000/100 = 10 pages.

• INL with outer not materialized.

� Decision not to push rating>5 before the join is based on availability of sid index on Sailors.

� Cost: Selection of Reserves tuples (10 I/Os); then, for each, must get matching Sailors tuple (1000*1.2); total 1210 I/Os.

� Join column sid is a key for Sailors.–At most one matching tuple, unclustered index on sid OK.

– Projecting out unnecessary fields from outer doesn’t help.

(On-the-fly)

(Use hashIndex, donot writeto temp)

Reserves

Sailors

sid=sid

bid=100

sname

rating > 5

(Index Nested Loops,

with pipelining )

(On-the-fly) What is needed for optimization?

• Iterator Interface• Cost Estimation• Statistics and Catalogs• Size Estimation and Reduction Factors

Summary• Query optimization is an important task in a relational

DBMS.• Must understand optimization in order to understand

the performance impact of a given database design (relations, indexes) on a workload (set of queries).

• Two parts to optimizing a query:– Consider a set of alternative plans.

• Must prune search space; typically, left-deep plans only.

– Must estimate cost of each plan that is considered.• Must estimate size of result and cost for each plan node.• Key issues: Statistics, indexes, operator implementations.

Query Optimization

• Query can be dramatically improved by changing access methods, order of operators.

• Iterator interface• Cost estimation

– Size estimation and reduction factors• Statistics and Catalogs• Relational Algebra Equivalences• Choosing alternate plans• Multiple relation queries• Will focus on “System R”-style optimizers

Highlights of System R Optimizer• Impact:

– Most widely used currently; works well for < 10 joins.• Cost estimation:

– Very inexact, but works ok in practice.– Statistics, maintained in system catalogs, used to estimate

cost of operations and result sizes.– Considers combination of CPU and I/O costs.– More sophisticated techniques known now.

• Plan Space: Too large, must be pruned.– Only the space of left-deep plans is considered.– Cartesian products avoided.

Query Blocks: Units of Optimization• An SQL query is parsed into a

collection of query blocks, and these are optimized one block at a time.

• Nested blocks are usually treated as calls to a subroutine, made once per outer tuple. (This is an over-simplification, but serves for now.)

SELECT S.snameFROM Sailors SWHERE S.age IN

(SELECT MAX (S2.age)FROM Sailors S2GROUP BY S2.rating)

Nested blockOuter block� For each block, plans considered are:

– All available access methods, for eachreln in FROM clause.– All left-deep join trees (i.e., all ways to join the relations one-at-a-time, with the inner reln in the FROM clause, considering all reln permutations and join methods.)

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Schema for Examples

• Reserves:– Each tuple is 40 bytes long, 100 tuples per page, 1000

pages. 100 distinct bids.• Sailors:

– Each tuple is 50 bytes long, 80 tuples per page, 500 pages. 10 Ratings, 40,000 sids.

Sailors (sid: integer, sname: string, rating: integer, age: real)Reserves (sid: integer, bid: integer, day: dates, rname: string)

Translating SQL to Relational Algebra

SELECT S.sid, MIN (R.day)FROM Sailors S, Reserves R, Boats BWHERE S.sid = R.sid AND R.bid = B.bid AND B.color = “red” AND S.rating = ( SELECT MAX (S2.rating) FROM Sailors S2)GROUP BY S.sidHAVING COUNT (*) >= 2

For each sailor with the highest rating (over all sailors), and at least two reservations for red boats, find the sailor id and the earliest date on which the sailor has a reservation for a red boat.

Translating SQL to Relational AlgebraSELECT S.sid, MIN (R.day)FROM Sailors S, Reserves R, Boats BWHERE S.sid = R.sid AND R.bid = B.bid AND B.color = “red” AND S.rating = ( SELECT MAX (S2.rating) FROM Sailors S2)GROUP BY S.sidHAVING COUNT (*) >= 2

Inner Blockπ S.sid, MIN(R.day)

(HAVING COUNT(*)>2 (GROUP BY S.Sid (B.color = “red” S.rating = val(Sailors Reserves Boats))))

σ∧

Relational Algebra Equivalences• Allow us to choose different join orders and to `push’

selections and projections ahead of joins.• Selections: (Cascade)

• This means we can do joins in any order.

( ) ( )( )σ σ σc cn c cnR R1 1∧ ∧ ≡... . . .( )( ) ( )( )σ σ σ σc c c cR R1 2 2 1≡ (Commute)

� Projections: ( ) ( )( )( )π π πa a anR R1 1≡ . . . (Cascade)

� Joins: R (S T) (R S) T≡ (Associative)

(R S) (S R) ≡ (Commute)

More Equivalences• A projection commutes with a selection that only uses

attributes retained by the projection.• Selection between attributes of the two arguments of

a cross-product converts cross-product to a join.• A selection on just attributes of R commutes with

R S. (i.e., σσσσ(R S) ≡≡≡≡ σσσσ(R) S )• Similarly, if a projection follows a join R S, we can

`push’ it by retaining only attributes of R (and S) that are needed for the join or are kept by the projection.

Cost Estimation

• For each plan considered, must estimate cost:– Must estimate cost of each operation in plan tree.

• Depends on input cardinalities.• We’ve already discussed how to estimate the cost of

operations (sequential scan, index scan, joins, etc.)– Must estimate size of result for each operation in tree!

• Use information about the input relations.• For selections and joins, assume independence of

predicates.– In System R, cost is boiled down to a single number

consisting of #I/O + factor * #CPU instructions– Q: Is “cost” the same as estimated “run time”?

Statistics and Catalogs• Need information about the relations and indexes

involved. Catalogs typically contain at least:– # tuples (NTuples) and # pages (NPages) per rel’n.– # distinct key values (NKeys) for each index.– low/high key values (Low/High) for each index.– Index height (IHeight) for each tree index.– # index pages (INPages) for each index.

• Stats in catalogs updated periodically.– Updating whenever data changes is too expensive; lots

of approximation anyway, so slight inconsistency ok.• More detailed information (e.g., histograms of the

values in some field) are sometimes stored.

Reduction Factors & Histograms

• For better estimation, use a histogram

equiwidthNo. of Values 2 3 3 1 8 2 1Value 0-.99 1-1.99 2-2.99 3-3.994-4.99 5-5.99 6-6.99

No. of Values 2 3 3 3 3 2 4Value 0-.99 1-1.99 2-2.99 3-4.05 4.06-4.67 4.68-4.99 5-6.99

equidepth

Size Estimation and Reduction Factors

• Consider a query block:• Maximum # tuples in result is the product of the

cardinalities of relations in the FROM clause.• Reduction factor (RF) associated with each term

reflects the impact of the term in reducing result size. • RF is usually called “selectivity”.

SELECT attribute listFROM relation listWHERE term1 AND ... AND termk

Result Size Estimation for Selections• Result cardinality =

Max # tuples * product of all RF’s.(Implicit assumption that values are uniformly distributed

and terms are independent!)• Term col=value (given index I on col )

RF = 1/NKeys(I) • Term col1=col2 (This is handy for joins too…)

RF = 1/MAX(NKeys(I1), NKeys(I2))• Term col>value

RF = (High(I)-value)/(High(I)-Low(I))

• Note, if missing indexes, assume 1/10!!!

Result Size estimation for joins

• Q: Given a join of R and S, what is the range of possible resultsizes (in #of tuples)?– Hint: what if R∩S = ∅?

– R∩S is a key for R (and a Foreign Key in S)?

• General case: R∩∩∩∩S = {A} (and A is key for neither)– estimate each tuple r of R generates NTuples(S)/NKeys(A,S) result

tuples, so…NTuples(R) * NTuples(S)/NKeys(A,S)

– but can also consider it starting with S, yielding: NTuples(R) * NTuples(S)/NKeys(A,R)

– If these two estimates differ, take the lower one!• Q: Why?

Enumeration of Alternative Plans• There are two main cases:

– Single-relation plans– Multiple-relation plans

• For queries over a single relation, queries consist of a combination of selects, projects, and aggregate ops:– Each available access path (file scan / index) is considered,

and the one with the least estimated cost is chosen.– The different operations are essentially carried out together

(e.g., if an index is used for a selection, projection is done for each retrieved tuple, and the resulting tuples are pipelined into the aggregate computation).

Cost Estimates for Single-Relation Plans

• Index I on primary key matches selection:– Cost is Height(I)+1 for a B+ tree, about 1.2 for hash index.

• Clustered index I matching one or more selects:– (NPages(I)+NPages(R)) * product of RF’s of matching

selects.• Non-clustered index I matching one or more selects:

– (NPages(I)+NTuples(R)) * product of RF’s of matching selects.

• Sequential scan of file:– NPages(R).

– Note: Must also charge for duplicate elimination if requried

Example

• If we have an index on rating:– Cardinality: (1/NKeys(I)) * NTuples(S) = (1/10) * 40000 tuples

retrieved.– Clustered index: (1/NKeys(I)) * (NPages(I)+NPages(S)) = (1/10) *

(50+500) = 55 pages are retrieved. – Unclustered index: (1/NKeys(I)) * (NPages(I)+NTuples(S)) = (1/10) *

(50+40000) = 4005 pages are retrieved. • If we have an index on sid:

– Would have to retrieve all tuples/pages. With a clustered index, the cost is 50+500, with unclustered index, 50+40000.

• Doing a file scan:– We retrieve all file pages (500).

SELECT S.sidFROM Sailors SWHERE S.rating=8

Queries Over Multiple Relations

• Fundamental decision in System R: only left-deep join trees are considered.– As the number of joins increases, the number of alternative

plans grows rapidly; we need to restrict the search space.– Left-deep trees allow us to generate all fully pipelined

plans.• Intermediate results not written to temporary files.• Not all left-deep trees are fully pipelined (e.g., SM join).

BA

C

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C

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C DBA

Enumeration of Left-Deep Plans• Left-deep plans differ only in the order of relations, the access

method for each relation, and the join method for each join.– maximum possible orderings = N! (but no X-products)

• Enumerated using N passes (if N relations joined):– Pass 1: Find best 1-relation plans for each relation.– Pass 2: Find best ways to join result of each 1-relation plan as outer to

another relation. (All 2-relation plans.) – Pass N: Find best ways to join result of a (N-1)-relation plan as outer

to the N’th relation. (All N-relation plans.)• For each subset of relations, retain only:

– Cheapest plan overall (possibly unordered), plus– Cheapest plan for each interesting order of the tuples.

A Note on “Interesting Orders”

• An intermediate result has an “interesting order” if it is sorted by any of:

– ORDER BY attributes– GROUP BY attributes– Join attributes of other joins

System R Plan Enumeration (Contd.)

• An N-1 way plan is not combined with an additional relation unless there is a join condition between them, unless all predicates in WHERE have been used up.– i.e., avoid Cartesian products if possible.

• ORDER BY, GROUP BY, aggregates etc. handled as a final step, using either an `interestingly ordered’ plan or an additional sorting operator.

• In spite of pruning plan space, this approach is still exponential in the # of tables.

• COST considered is #IOs + factor * CPU Inst

Example

• Pass1:Reserves: B+ tree on bid matches bid=100.Sailors: B+ tree matches rating>5, but

this selection is expected to retrieve a lot of tuples, and index is unclustered, so file scan w/ select is likely cheaper.

Reserves:Unclust B+ tree on bid

Sailors:Unclust Hash on sidUnclust B+ tree on rating

Pass 2:We consider each Pass 1 plan as the outer:Reserves as outer (B+Tree selection on bid):

Use hash index on Sailors.sid for INL Sailors as outer (File Scan w/select on rating):

Use BNL on result of selection on Reserves.bid

Reserves Sailors

sid=sid

bid=100 rating > 5

sname Example Sailors:Hash, B+ on sid

Reserves:Clustered B+ tree on bidB+ on sid

BoatsB+, Hash on color

Select S.sid, COUNT(*) AS numbesFROM Sailors S, Reserves R, Boats BWHERE S.sid = R.sid AND R.bid = B.bid

AND B.color = “red” GROUP BY S.sid

Reserves

Sailors

sid=sid

Boats

Sid, COUNT(*) AS numbes

GROUPBY sid

bid=bid

Color=red

• Pass1: Best plan(s) for accessing each relation– Reserves, Sailors: File Scan– Q: What about Clustered B+ on Reserves.bid???– Boats: B+ tree & Hash on color

Pass 2• For each of the plans in pass 1, generate plans joining

another relation as the inner, using all join methods– File Scan Reserves (outer) with Boats (inner)– File Scan Reserves (outer) with Sailors (inner)– File Scan Sailors (outer) with Boats (inner)– File Scan Sailors (outer) with Reserves (inner)– Boats hash on color with Sailors (inner)– Boats Btree on color with Sailors (inner)– Boats hash on color with Reserves (inner) (sort-merge)– Boats Btree on color with Reserves (inner) (BNL)

• Retain cheapest plan for each pair of relations plus cheapest plan for each interesting order.

Pass 3 and beyond

• For each of the plans retained from Pass 2, taken as the outer, generate plans for the inner join– eg Boats hash on color with Reserves (bid) (inner) (sortmerge))

inner Sailors (B-tree sid) sort-merge

• Then, add the cost for doing the group by and aggregate:– This is the cost to sort the result by sid, unless it has

already been sorted by a previous operator.• Then, choose the cheapest plan

Nested Queries• Nested block is optimized

independently, with the outer tuple considered as providing a selection condition.

• Outer block is optimized with the cost of `calling’ nested block computation taken into account.

• Implicit ordering of these blocks means that some good strategies are not considered. The non-nested version of the query is typically optimized better.

SELECT S.snameFROM Sailors SWHERE EXISTS

(SELECT *FROM Reserves RWHERE R.bid=103 AND R.sid=S.sid)

Nested block to optimize:SELECT *FROM Reserves RWHERE R.bid=103

AND R.sid= outer valueEquivalent non-nested query:SELECT S.snameFROM Sailors S, Reserves RWHERE S.sid=R.sid

AND R.bid=103

Points to Remember

• Must understand optimization in order to understand the performance impact of a given database design (relations, indexes) on a workload (set of queries).

• Two parts to optimizing a query:– Consider a set of alternative plans.

• Must prune search space; typically, left-deep plans only.

– Must estimate cost of each plan that is considered.• Must estimate size of result and cost for each plan node.• Key issues: Statistics, indexes, operator implementations.

Points to Remember

• Single-relation queries:– All access paths considered, cheapest is chosen.– Issues: Selections that match index, whether index key

has all needed fields and/or provides tuples in a desired order.

More Points to Remember

• Multiple-relation queries:– All single-relation plans are first enumerated.

• Selections/projections considered as early as possible.

– Next, for each 1-relation plan, all ways of joining another relation (as inner) are considered.

– Next, for each 2-relation plan that is `retained’, all ways of joining another relation (as inner) are considered, etc.

– At each level, for each subset of relations, only best plan for each interesting order of tuples is `retained’.

Summary

• Optimization is the reason for the lasting power of the relational system

• But it is primitive • New areas: Rule-based optimizers, random

statistical approaches (eg simulated annealing)

review relational query optimization

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